3.8 POND SYSTEM MAINTENANCE

3.8.1 Desludging

Research has shown that management of sludge levels in the anaerobic pond is a key factor affectingaerobic pond performance.

If an analysis has been taken of the pond outflow and the quality is termed poor, this may indicate that the pondrequires desludging. However, farmers should not wait until such a situation exists before desludging a pond.

Effluent flow will be reduced in ponds that are not regularly desludged. The outlet baffle (i.e. subsurface pipe,T-piece or timber baffle) may clog up and fall into disrepair due to sludge build-up or excessive crusting.

3.8.1.1 When to desludge

Regional Councils have different rules and consent conditions regarding desludging. It is recommended that as aminimum, anaerobic ponds should be desludged at least once every four years but best practice is todesludge every two years or even annually.

Desludging is less of a concern to Regional Councils if the pond is used for storage before land application, ratherthan discharging to a waterway.

Undersized ponds, those in colder climates and those that discharge into sensitive environments need morefrequent desludging. In-flows to ponds will also influence the need for desludging e.g. if a stand-off area or feedpad is connected to the effluent system.

A useful indicator of when individual ponds may need desludging is when the sludge level is over half the normaleffluent depth. This can be checked by probing with a long pole.

The aerobic pond will need desludging less frequently, depending on the rate of sediment build up.However, it may be more convenient to desludge all ponds in the system at the same time.

The consumption of poorer quality pasture by the herd (e.g. pastures high in kikuyu, paspalum and fescue) mayresult in excessive crusting and require more frequent desludging.

If operating barrier ditches the system, especially the first ditch sections, will need regular desludging. If they arenot desludged regularly, the system will become inefficient and ineffective at treating the effluent. Effluent flowwill be reduced and the outlet baffles may clog up and fall into disrepair due to sludge build-up or excessivecrusting. As a general rule, barrier ditches should be desludged annually.

3.8.1.2 Methods of desludging

Sludge and crusts are usually removed with excavation machinery. Alternatively, a drag line can be used.Contractors in most regions have appropriate equipment and experience in desludging ponds.

If possible, the surface liquid effluent should be removed before desludging by suction drawing it into a vehiclespreader (refer to 2.10 Vehicle spreading).

Removal of the liquid effluent makes the sludge easier for excavators to contain and handle. The liquid can bepumped out using a vehicle spreader, where a tanker is backed close to the pond and effluent is drawn into it by aPTO-driven pump.

Pond stirring, to mix the various layers of the pond during tanker filling, can remove the need for excavators todesludge ponds (refer to Figure 3.8-1).

Effluent from the aerobic pond can also be pumped into the first pond to make it easier to dilute and removeeffluent solids prior to land application.

Ponds should never be emptied out completely. A third of the sludge should be left behind as it will containbacterial populations necessary for the continuation of anaerobic processes.

3.8.1.3 Applying pond sludge to land

Considerations when applying pond sludge to land include:

• regional council regulations regarding land application of effluent, and in particular the higher concentrationof nutrients in sludge

• buffer distances from waterways and public areas

• correct application rates and area requirements

• suitable weather and ground conditions.

For information on the application of pond sludge to land refer to 2. Land application and 2.8 Land application ofsludge.

3.8.1.4 Chemical and biological treatments

Several kinds of additives are available and are claimed to dissolve and disperse solids build-up, particularlycrusting. These additives vary in their effectiveness. Current evidence shows that they are not a long-term solutionto reducing sludge and crusting problems.

3.8.2 Weed control

Crusts on ponds may begin to support vegetative growth (refer to Figure 3.8-2). This can give the appearance ofsolid ground, creating a potential safety hazard for people and stock. Furthermore, the roots of weeds will entanglepond stirrers and interfere with desludging machinery. Do not let weeds build up on the crust layer of ponds.

However, maximise weed growth in receiving drains. The weeds will act as a filter, taking up effluent nutrientsand sifting out suspended solids. Clean out only a third of the receiving drains at a time so that there is alwaysweed present to assist with effluent filtering.

Do not spray the drains.

3.8.3 Daily

• Before and after every milking, check that the stormwater or washwater diversion is in the correct position.

3.8.4 Regularly

• Clean and clear the effluent stone trap and gratings.

• Check that the pipes running in and out of the ponds are not blocked.

• Check the effect of the discharge on the receiving waterway.

• Check that the pond walls are stable, and that there is no seepage. Visible wetness or pasture that isgrowing exceptionally well are indicators of seepage problems.

• Control weed growth in and around ponds by spraying with a herbicide.

• Check that the fencing remains stock-proof.

3.8.5 Six monthly to annually

• When the area around the ponds is dry, graze them.

• Clean out only a third of the receiving drains at a time so that there is always weed present to assist witheffluent filtering.

• Check that there is not excessive build-up of solids in the anaerobic pond.

• Desludge ponds regularly, preferably annually or every two years but at least once every four years.Dispose of sludge on to land according to recommendations in 3.8.1.3 Applying pond sludge to land.

• Check that the pond is not becoming shaded by vegetation as this will decrease treatment efficiency.

3.9 POND SYSTEM REGULATIONS

Pond systems are designed and constructed relative to the volume and quality of farm dairy effluent they areexpected to handle, and the region in New Zealand in which they are sited. However, Regional Councils (andDistrict Councils) and the NZ Food Safety Authority in conjunction with the Dairy Industry also haveregulations governing the design of pond systems.

3.9.1 Food safety and dairy industry requirements

The health and hygiene practices on the farm, and within the farm dairy, are closely monitored by overseas marketsand have a considerable effect on the saleability of New Zealand’s dairy products. Hence, the Dairy Industry hasworked with the NZ Food Safety Authority to put hygiene regulations in place that must be followed to helppromote the industry to overseas consumers.

These regulations are focused on human and animal health as effluent may contain transmissible animal diseases,including bacteria, viruses, cysts, and eggs and larvae of parasites (e.g. hookworm, roundworm and tapeworm).

Disease-causing micro-organisms present in effluent originate mainly from stock and so the levels of disease-causing microorganisms reflect the current state of health of the herd. Therefore, with good husbandrypractices, effluent originating from dairy cows should be free of major diseases.

If effluent is retained within the pond system for long enough pathogenic micro-organisms will bedestroyed.

The survival of various disease-causing micro-organisms during effluent storage and treatment is summarised in2.13.1 Food safety and dairy industry requirements.

3.9.1.1 Food safety, dairy industry and health regulations

Food Safety regulations for the dairy industry (i.e. the Farm Dairy Code of Practice NZCP1) require that storagesumps (where the effluent is not immediately pumped or gravity fed into ponds) must not be located within 10 m of the milking area, milk receiving area or milk storage area. Sumps greater than 22500 L must not bewithin 45 m of the above areas.

Effluent may not be disposed of within 45 m of the farm dairy (i.e. milking area, milk receiving area and milkstorage area). This includes pond systems, barrier ditches, constructed wetlands and places where effluent isapplied to land.

Occupational Safety and Health regulations require stock to be vaccinated against harmful animal andhuman diseases (e.g. leptospirosis).

Care should be taken with personal hygiene after handling effluent in any form, and when working with treatedsoil.

3.9.2 Regional Council requirements

Regional Council concern is primarily focused on:

• siting, design and construction of pond and barrier ditch systems

• the quality and quantity of the discharge

• environmental effects that the discharged effluent may have on waterways.

3.9.2.1 Pond siting, design, construction and operation

In light of the Resource Management Act (1991), Regional Councils have policies and rules in their Regional Plansthat describe what effluent and receiving water quality standards must be met, rather than outlining thephysical design of the system itself.

Regional Councils may also insert conditions in resource consents regarding the design of pond systems. Thefollowing is a list of those components of the system that must be taken into account in design to avoid orminimise adverse effects. Some of these aspects may also be standards for design and specified in Council rules orresource consents.

• Stormwater control. Stormwater originating from the farm dairy roof or yards, or from runoff from the landsurrounding the pond system, will add loading and flush the pond system. This has the effect of decreasingeffluent retention time and, subsequently, treatment time, effluent quality and efficiency.

• Solid waste entry prevention. Offal, pesticides, fertiliser and rubbish can block the treatment system as well asreduce the system's ability to treat the effluent.

• Receiving water restrictions. Certain surface waterways are too sensitive to receive treated effluent at anytime (e.g. fresh water lakes and natural wetlands). The concentration of ammonia in receiving waters may bemeasured, following reasonable mixing, to determine the potential for waterway pollution.

• Siting. Buffer distances between ponds and surface waterways and groundwater are necessary so that the riskof contaminating water is minimised. Locations such as swampy ground and sloping ground may be subject toinstability and stormwater or groundwater intrusion into the system.

• Pond sizing. The size of both the anaerobic pond and the aerobic pond is usually based on the BOD loadingfrom maximum herd numbers. The depth of the ponds is important as system efficiency is largely determinedby the pond depth.

• Freeboard. The minimum freeboard is given as insurance against flash loading and to reduce the erosioneffects of wave motion.

• Embankment construction. The slope of batters, presence of key trenches, and the methodology in laying andcompacting soil all affect the stability of the pond structure and its ability to withstand erosion and seepage.

• Sealing. Ponds need to be sealed so that no effluent seeps to groundwater, or through the embankment tosurface water. Most Councils specify that ponds must be impermeable.

• Inlet and outlet structures. The design and position of the inlets and outlets can determine the efficiency ofthe system. Incorrect installation may result in seepage, embankment erosion, short-circuiting of the ponds andsystem blockages.

• Fencing. Fences are essential for the protection of stock and farm labour, and the protection of embankmentsfrom animal damage.

• Desludging. Over-accumulation of sludge or crusting decreases the treatment efficiency of effluent andincreases the risk of solids passing into the system. However, the complete removal of the sludge layer can haltthe operation of the pond system because of the absence of beneficial anaerobic bacteria populations living inthe sludge.

• Weed control. Weed growth adversely affects the operation of the pond system as well as causing a physicalnuisance with pipe blockages.

• Shading. The ponds (most importantly the second and any subsequent ponds) should not be shaded by treesor structures that can reduce the treatment of the effluent.

3.9.2.2 Measurable effects of effluent discharged into waterways

Regional Councils are responsible for the monitoring and upkeep of water quality standards. Table 3.9-1 shows alist of those factors which may be measured and used as standards for discharges to waterways and some typicalcritical levels for water quality (refer to 1.4 Why control the discharge of effluent? and 5.2 Resource managementact (1991)).

Ministry of Health, 1995; Department of Health, 1984; Department of Health, 1992; Ministry of Agriculture and Fisheries, 1993; Parminter, 1995.

3.9.2.3 Regional Council regulations regarding effluent discharge to surface water

Each Regional Council has different rules for the discharge of effluent to surface water. Regional Councils may alsohave different rules between catchments within the same region (check with the Regional Council for the relevantrules). Table 3.9-2 outlines how the activity is classified in each region. A permitted activity has certain conditionsthat must be met but does not require a resource consent, whereas a controlled, discretionary, restricteddiscretionary or non-complying activity all require a consent before the activity can commence. A prohibitedactivity means that it will not be possible to get a consent for the discharge of effluent to surface water. For moreinformation on the different classifications of activities refer to section 5.3.1 What is a regional plan? If theconditions of the activity cannot be met then the classification is likely to be more restrictive. For example if theconditions of a controlled activity rule cannot be met then it is likely to be classified as a discretionary or non-complying activity.

Table 3.9-2 is a summary and should not be used for legal purposes. To obtain the information in full, request theRegional Plan(s) pertaining to soils, water and air quality from the Regional Council. The classifications of theseactivities are current (January, 2006) and may be subject to change by the Regional Councils.

3.10 CONSTRUCTED WETLANDS

Wetlands are areas which support the growth of a variety of plant species adapted to flooded conditions for part of,or the entire, year. The plants are densely spaced and, together with the shallow water, provide good wildlifehabitat as well as water purification.

Constructed wetland systems are designed to simulate and optimise the filtering and organic matter breakdownprocesses that occur in natural wetlands. They are a possible solution to improve the performance of pondsystems, as they can ‘polish’ farm dairy effluent before discharge to a waterway.

During summer months, such a system may even result in zero discharge to waterways, due to evapotranspirationof water from the wetland.

Constructed wetlands are designed to ensure greater reliability, a low risk of environmental impact and increasedcontrol over the treatment process. The advantages of constructed wetlands are that they:

• have the ability to treat a wide range of contaminants

• have the ability to handle shock loadings of effluent

• do not normally rely on electricity or machinery

• have aesthetic value and provide wildlife habitat

• are more acceptable to Maori than direct discharge to surface waterways

• require minimal ongoing capital expenditure and maintenance once established

• operate successfully over a wide range of climate regimes (as long as cold temperatures do not affect thefunctioning of the pre-treatment ponds).

Constructed wetlands are not 'stand alone' treatment systems. They are designed to polish effluent flowingfrom a pond system before it reaches a surface waterway.

For constructed wetlands to operate successfully the pond system must be working to expectations andeffluent flowing from the pond system must be treated to a high standard.

3.10.1 Planning for a constructed wetland system

Figure 3.10-1 gives the steps to follow when considering installing a constructed wetland as an additionaltreatment system.

3.10.2 Constructed wetland management

To ensure the best treatment of effluent, both the initial pond treatment system and the constructed wetlandshould be designed, constructed and planted correctly from the beginning. Constructed wetlands should notbe considered as a ‘patch up’ system for ponds or barrier ditches that are not working properly. Wetlands aredesigned to function effectively only if used together with an initial treatment system that is operating adequately.

Establishing and maintaining constructed wetland plants is the critical and most difficult challenge. To achieve ongoingplant growth, the constructed wetland must be maintained regularly and properly from the time of installation.

Control of effluent levels within the wetland is also important during plant establishment. After planting keepeffluent levels to a minimum (i.e. 50 mm to 100 mm) then gradually raise them as the plants develop (refer toFigure 3.10-2 and Figure 3.10-3).

Waterfowl (e.g. pukeko and canada geese) can pull out and destroy young plants. Electric fencing or otherdeterrents may be required during the establishment phase if these birds are present.

If contractors are involved in establishing the wetland plants, make it their contractual responsibility to have a setpercentage of successful and active growth by a certain time.

Maintenance involves monitoring, controlling pests and weeds, managing the plant species and repairing andmaintaining pipes and embankments.

3.10.3 Costs of a constructed wetland

A basic cost for installing a 400 m2 constructed wetland (surface flow) suitable for basic treatment ofoxidation pond effluent from 200 cows will be around $12 000 - $15 000 including earthworks, a clay liner,inlet and outlet structures, gravel and plants. Additional establishment costs (site survey, design andresource consent processes) may be up to $2500. Other costs depend on:

• the site. Obstacles such as rocks, the soil type and accessibility largely influence the time and effort required toexcavate soil and seal the wetland base

• whether any pumping facilities to and from the wetland are required

• the need for artificial liners.

The expense is governed by the complexity of the system.

3.10.4 Constructed wetlands as an economic and practical option

The financial costs associated with capital outlay, ongoing maintenance and labour requirements for a constructedwetland are moderate. Furthermore, effluent discharged to waterways from an effective wetland is of a higherquality. Together, this makes incorporating a wetland into the existing/planned pond system an attractive option,especially if habitat and aesthetic improvements are valued.

For both existing and planned pond systems a constructed wetland can be introduced in an effort to obtain boththe economic advantages of employing a pond system, and effluent quality standards that will satisfy RegionalCouncils and be of benefit to the environment.

3.10.5 How constructed wetlands work

Wetland processes treat waste in a number of ways:

• solid particles settle out in shallow, slow-flowing waters with dense vegetation. Algae production is alsoinhibited due to shading of the wetland plants. This is advantageous as algae growing within the effluent addto its BOD

• pathogenic micro-organisms are reduced through enhanced sedimentation (settling out with the solids),natural die-off and grazing by protozoa

• micro-organisms break down the wastes. Bacteria, fungi, protozoa and algae break down the organic matter.In particular, microbial slimes (biofilms) which form on the surface areas provided by plants, settled solids, soils,and gravel rapidly break down organic materials and take up nutrients. Wetlands provide a mosaic of oxygenated(aerobic) and oxygen-free (anaerobic) ‘micro-environments’ where different bacteria can work to break downmatter and transform nutrients such as nitrogen and sulphur to gases that are released back to the atmosphere.This is assisted by the snorkel-like action of wetland plants, which transport oxygen down to the root zone

• plants take up nutrients for their own growth and return them back to the wetland as organic matter whenthey die. This is largely a recycling system and will not account for much nutrient removal. In general, nutrientuptake and storage by plants generally accounts for a small proportion of nutrient removal from the wetland (5-15% on an annual basis).

The effective ‘polishing’ of effluent in a wetland system depends on the design employed for maximising retention,the quality of the in-flowing effluent, the climate and season, and the plants used.

An established wetland can be expected to behave differently than a recently constructed wetland.

The two principal types of constructed wetland systems are surface-flow wetlands and subsurface-flow wetlands.

Surface-flow wetlands have effluent flowing through the stems and lower foliage of the wetland plants that arerooted in soil. Subsurface-flow wetlands have effluent percolating horizontally through the root zone of thewetland plants growing in a gravel bed.

A combination of both surface-flow and subsurface-flow systems can also be employed. This combination, whencorrectly loaded and designed, can significantly improve on pond systems. The figures for wetland effectivenessvary depending on whether a single wetland is used (lower range figures below) vs a combination of surface andsubsurface (higher range figures below). The higher range nitrogen removal will only be achieved if mechanicalaeration is part of the pre-treatment pond system.

Above and beyond pond treatment, wetlands can achieve a:

• reduction of 35% to 75% BOD and suspended solids

• reduction of 15% to 80% total nitrogen through nitrification and denitrification processes and someammonia volatilisation. The higher rates of nitrogen removal can only be achieved with mechanical aeration inthe pond or intermittient - dose vertical flow wetlands

• reduction of 70 to 95% of pathogenic bacteria populations by filtration, biological breakdown and planttoxin effects. However, even at the high end of microbial treatment, the outflow will still usually containmicrobial contamination that exceeds 500 faecal coliforms/100 ml and therefore is not suitable for contactrecreation

• possible reduction of small and variable quantities of phosphorus and sulphur. Initial removal may behighest (i.e. up to 75%) with continuing removal depending on the use of special substrates to absorbphosphorus and management steps such as plant harvesting and wetland dredging. Ongoing phosphorusremoval could be higher if specifically absorbent soils or sediments are used (e.g. iron oxides, steel wool, bauxiteclays, allophane clays, natural loam soils, bauxite, red mud, red sand, smelter slag). However, the use of thesematerials has not yet been sufficiently researched to make recommendations.

3.10.5.1 Surface-flow wetlands

In surface-flow wetlands (refer to Figure 3.10-4) a slime layer or bio-film develops on the plant stems and in thelitter. This layer provides a habitat for bacteria that biologically break down effluent. Plants also filter outsuspended solids from the effluent stream.

Surface-flow wetlands are simpler to design, and less costly to construct than subsurface-flow wetlands.They can also tolerate higher suspended solids loadings and are more easily desludged and re-established.

Surface-flow wetlands are readily adapted to provide a habitat for wildlife. Open, unvegetated zones can becreated by providing additional deep zones within the wetland. However, effluent from the open area should passthrough a densely vegetated shallow zone before discharge to a waterway.

3.10.5.2 Subsurface-flow wetlands

A subsurface-flow constructed wetland (i.e. ‘root-zone’ system) works similarly to ‘hydroponics’. The wetland bed isexcavated and lined with an impermeable layer. It is then backfilled with the gravel in which the plants root andthere is no free water above the gravel surface (refer to Figure 3.10-5).

A slime layer develops on the gravel and plant root rhizomes, providing a habitat for bacteria and other microbesthat break down effluent.

Subsurface-flow wetlands require more accurate and detailed engineering design than surface-flow wetlandsystems to ensure correct gradient and flow. They are more costly because suitable gravels have to be broughtonto the site.

However, subsurface-flow wetlands can treat effluent to a much higher and consistent quality. Subsurface-flow wetlands have a high potential for suspended solid and nitrogen removal. They also have less potential forcausing nuisance with insects and odours since there is no exposed water surface.

3.10.5.3 Combined systems

To reduce the potential for clogging by suspended solids, surface-flow and subsurface-flow constructedwetlands can be used in combination.

Subsurface-flow wetlands are vulnerable to clogging if loaded with high rates of suspended solids, or subject tosediment inputs from stormwater or the erosion of embankments. Since effluent flowing from a pond systemhas variable and often high levels of suspended solids, subsurface-flow wetlands are best preceded by asurface-flow system.

The surface-flow wetland can reduce levels of suspended solids that may otherwise clog up the subsurface-flowsystem, and can provide initial BOD reduction and a reduction of other contaminants. It can be desludged whenaccumulation occurs.

3.10.6 Siting of constructed wetlands

The availability of land in an appropriate location often proves to be the limiting factor when considering theinstallation of a constructed wetland facility. (Refer to 3.4 Siting of ponds, as this section discusses considerationswhen siting effluent treatment systems.)

Wherever possible, wetlands should be constructed below the initial treatment system so that gravity canbe used to convey the effluent.

The contour of the land needs to be selected on the basis of the most cost-effective wetland cell arrangement.Such an arrangement should minimise earthworks and loss of grazing area while achieving the treatmentrequirement and best fit with the existing landscape. Consider surface and subsurface drainage, the location ofsprings and potential for flooding. Avoid springs and steep slopes, as stormwater runoff will add loading to thesystem, and retention of water in the wetland may be too short. Consider the outflow point and where thedischarge will go.

The availability of suitable soil for the construction and sealing of the wetland is important (i.e. soil withsignificant day content). Suitable soil is also required for the growth of plants.

Constructed wetlands rarely cause odour and insect nuisances. However, when planning the location ofwetlands it is still wise to consider the risk of such issues (refer to 3.4.3 Wind direction and proximity to residentialhousing). As an effluent treatment system, food safety regulations apply so the wetland should not be within 45 mof a farm dairy (refer to 2.13.1 Food safety and dairy industry requirements).

3.10.7 Wetland design and construction

Constructed wetland design does not lend itself to conventional design criteria. This is because there are site-specific details such as the standard of farm dairy effluent pre-treatment, the desired standard of wetlandtreatment and suitable local wetland plant species.

This chapter addresses the general design of constructed wetland systems.

Comprehensive constructed wetland design guidelines were released by NIWA in 1997 (see the NIWA websitewww.niwa.cri.nz). These guidelines should be followed, along with appropriate advice from expert designers.

In constructed wetland design, the key variables on which to focus are the:

• effluent volume and loading

• retention time

• size and configuration

• vegetation.

3.10.7.1 Effluent loadingThe first step in constructed wetland design is to identify the treatment objectives. This is done by identifying:

• the required quality standards of the discharged effluent. Regional Councils have rules regulating thedischarge of effluent into waterways (check with your Regional Council for requirements)

• the original volume and characteristics of the effluent and degree of pre-treatment that the effluent hasalready undergone in the pond system (refer to 1.6.1 Effluent characteristics and volumes)

The required standards at outflow and percentage decrease in key contaminants of the pond effluent willdetermine the wetland size and whether a combination of wetlands is required.

Table 3.10-1 shows the approximate percentage of removal for key effluent contaminants for a single wetlandtreatment option and a combination option of a surface-flow followed by subsurface-flow wetland.

Note: Assumes both pre-treatment and wetland systems are functioning well and normal effluent volumes apply.Adapted from Tanner and Kloosterman, 1997.

To improve pre-treatment:

• correctly carry out pond system maintenance (refer to 3.8 Pond system maintenance)

• ensure the outflow pipe and baffle system from the aerobic pond is allowing only liquid effluentthrough. Take effluent from below the pond surface where algae are concentrated, but above any sludge layeron the pond floor

• provide pre-filtration of effluent through rock filters

• provide mechanical aeration in the pond. This will greatly reduce total nitrogen and ammonia content in thewetland discharge.

The outflow from the wetland will be low in dissolved oxygen. Oxygenation can be enhanced by cascading theeffluent over land or rocks before discharge to a waterway.

Having considered the effluent loading in and out of the system, the remaining design process revolves aroundobtaining maximum retention time, simple operation and minimal maintenance.

3.10.7.2 Retention time

Treatment efficiency of constructed wetlands is generally related to retention time.

Effluent should be maintained within the constructed wetland system for 7 to 14 days. Shorter times thanthis will result in poor performance.

For surface-flow wetlands, an acceptable retention time is 7-10 days.

Where a combined surface-flow and subsurface-flow system is employed to provide higher levels oftreatment, the effluent should remain in the initial section for 7 - 12 days and the subsurface-flow sectionfor 2-3 days.

3.10.7.3 Size and configuration

The size of the constructed wetland is determined by the herd size contributing to the volume of effluent flow, theaverage depth of the wetland and the retention time.

The depth of the wetland is largely dependent on the plants chosen and controlled by the use of adjustable swivelpipe outlets. However, it is usually between 300 mm and 400 mm deep.

Table 3.10-2 gives recommended bed surface areas for dairy farm wetlands to achieve the treatment levels given inTable 3.10-1. These surface areas do not include the area required for the embankments. For larger herds, thesedimensions may be divided among multiple cells.

Note 1: Effluent volumes based on 50 l/cow/day (refer to 1.6.1 Effluent volumes and characteristics).Note 2: Assuming the surface-flow wetland has an average depth of 300 mm and a 10-day retention time.Note 3: Assuming a combined wetland with the surface-flow section of 300 mm average depth and a 12-day retention time, and the

subsurface-flow section of 400 mm average depth and a 3-day retention time.Note 4: Assuming the mature vegetation, litter and sludge takes up 20% of the wetland volume.Note 5: Assuming the gravel in the subsurface-flow system takes up 65% of the wetland volume.

* For these larger sizes, division into two parallel cells is recommended.

3.10.7.4 Multiple cells

The use of multiple channels or cells, rather than a single large wetland, is recommended on properties withherds of more than 200 cows to obtain maximum flexibility and facilitate maintenance. Two or three cells areadequate.

In such designs, cells can be isolated and closed down for maintenance, or rested when flows are low, withoutaffecting the overall system. The number of cells will depend on the total size of the wetland system and siteconstraints. Addition of further cells can accommodate herd size increases.

3.10.7.5 Length to width ratios and shape

Appropriate length to width ratios will maximise treatment efficiency. Ratios of between 4 : 1 and 6 : 1 ( i.e. 4 m to6 m length to every 1 m width) are recommended for surface-flow systems and 2 : 1 for subsurface-flowsystems. Such ratios will avoid excessive loading at the inlet end while still promoting uniform flow of effluentacross the width of the wetland rather than ‘short-circuiting’ of the system.

For subsurface-flow systems, the saturated cross-section of the bed must be sufficient to contain the design flow.Problems can occur if the beds are very long and narrow, or media with a low hydraulic permeability are used (e.g.soil or sand).

The shape of wetlands can be curved slightly to provide a more natural fit with the landscape. To ensure efficienttreatment, only gentle curves should be used to avoid creating stagnant backwaters. Widths should not vary morethan 20% for surface-flow wetlands and 10% for subsurface-flow wetlands.

3.10.7.6 Excavation and embankment construction

Wetland cells are best formed and separated by earthen embankments. These serve to contain flows andcontrol the effluent level. Timber dams should not be used for dairy farm wetland systems.

Recommended equipment for construction of embankments are a hydraulic excavator for earth moving andplacement, and a rubber-tyred tractor towing a sheepsfoot roller for compaction. Embankment materials should bespread and compacted in layers of no more than 200 mm thickness.

For cell embankment construction refer to Figure 3.10-7 and Figure 3.10-8. The following design guidelines arerecommended:

• a freeboard of 300 mm

• the top on the embankment separating the wetland cells should be at least 2.5 m wide. Widths can beincreased to aid construction and also allow for maintenance machinery access if side access is a problem.Where soil is porous or the wetland is to be built above ground, key trenches are required (refer to 3.6Construction of ponds)

• the batter slope on the embankments should be 3 horizontal to 1 vertical. This will ensure stability and allowmachinery access to the wetland’s edge (refer to 3.6.2 Batters). Inner embankments can have a gradient of 2:1.

3.10.7.7 Stormwater control

In some situations wetlands may be useful for the treatment of water runoff from land. Since such water may containlarge quantities of nutrients from the farm system, the wetland will provide a buffer between the land and waterways.

However, during high volume stormwater runoff from surrounding land, where the capacity of the wetland isexceeded, large increases of pollutant loading can occur. This is due to:

• additional pollutants within the stormwater such as soil, nutrients and pesticides

• the incoming water disturbing already settled solids. The solids, containing nutrients, are re-suspendedwithin the effluent.

Therefore, there should be a diversion channel, or cut-away ditch, around the top of the wetlands to divertsurface runoff (refer to 3.6.3.2 Diversion channels around the ponds).

3.10.7.8 Inlet and outlet design

The inlet and outlet structures should allow for water level control and complete draining of the cells formaintenance (refer to Figure 3.10-8). Flow velocities within the wetland system must be low to promote thesedimentation and accumulation of solids. Levels will need to rise as vegetation is established and as sedimentbuilds up in the mature wetland system.

Baffled outlet structures from the initial pond treatment system should take in effluent from well below the pondsurface to reduce the proliferation of algae in the wetland system. The effluent may be collected at an intermediarycollection point before distribution to the wetland cells (refer to Figure 3.10-6).

The inlet system is designed to achieve even distribution of the flow across the entire wetland width. It isimportant that the inlet pipe is laid level so that inflows are dispersed evenly across the bed using adjustable T-pipes for final distribution of flows (refer to Figure 3.10-8).

The outlet structure can use a swivelling outlet pipe system to allow simple water level adjustment with lowclogging potential. The swivel pipe should be contained within an enclosed perforated concrete sump unit ortreated timber box, set in a coarse gravel zone (refer to Figure 3.10-8).

Final discharge from a wetland can be via simple overland flow, seepage fields, vegetated drains or naturalwetlands. These are preferable to direct discharge to streams (refer to 3.6.5.5 Outlet pumping, drains orwetlands).

Pristine, low nutrient bogs or wetlands with high conservation values should not be used to receive a constructedwetland discharge, but most wetlands dominated by raupo on farmland will readily assimilate this type ofwastewater. Check the policies of your Regional Council before proceeding with this option.

If the discharge is to a natural waterway, flowing the water over simple rock cascades or waterfalls can help aerateit, addressing the low dissolved oxygen levels that can occur in constructed wetland discharges.

Effluent can be dispersed through a perforated pipe laid along the contour of a gentle slope (2-10% gradient),allowing water to flow for at least 5-10 m before reaching a waterway. A pipe 20 m in length with 20 mm holesdrilled at 200 mm spacings would be required for an average 200 cow herd. Removable end-caps can be used toallow periodic flushing of the dispersal pipe.

It is best if small intermittent doses are applied rather than continuous flows (e.g. 1 hours application followed by 3-4 hours rest). It is also good to have two or more disposal areas (e.g. 1 year’s application followed by 1 year’s restperiod).

The dispersal area should be fenced from stock to avoid pugging.

3.10.7.9 Sealing and lining

Sealing and lining is necessary for wetlands. Sealing ensures the effluent is not lost from the wetland systemthrough seepage.

Clay can be used to line the wetland and should be kept moist to avoid splitting. Test that the wetland iswatertight before adding topsoil and planting.

Heavy-duty polyethylene lining (greater than 1 mm) is necessary where clay soils are not available for compactingand sealing. Such a liner will be subject to at least 200 mm of soil or gravel being placed upon it for plant rootingpurposes.

Since some polyethylene liners are not manufactured to sustain such loadings, a geotextile or butyl rubber linercan be used as its strength will protect it from soil and gravel damage. These liners are also useful on the wetlandedges where sunlight may otherwise degrade polyethylene.

The liner should be placed over a layer of sand for cushioning and protection from stones. For further sealing andlining techniques and costings refer to 3.6.4 Sealing and lining.

3.10.7.10 Wetland media

In a surface-flow wetland, topsoil is spread to a depth of 200 mm and lime incorporated to reduce toxicity to plantsafter flooding. The soil should be carefully levelled and lightly compacted to avoid problems with patchy plantestablishment.

Uncompacted gravel is used in subsurface-flow wetlands, to a depth of 0.5 m. The top 150 mm of this should be alevelled layer of 20/12 mm gravel for plant growth with the lower layer being 65/40 coarser gravel, with no fines.

Both surface-flow and subsurface-flow wetlands need a gravel zone around the inlet and outlet structures of coarse65/40 mm clean, hard gravel with no fines.

Gravel size and hardness are critical, as the media can be easily clogged. Avoid any soil entering the gravel as thiscan also cause clogging.

3.10.7.11 Vegetation

Constructed wetland vegetation:

• assists in flow regulation and promotes settling of solids

• provides surfaces for bacterial films which carry out effluent treatment

• provides oxygen to the aerobic bacteria byreleasing it to the root zone

• takes up nutrients

• shades the wetland from sunlight, reducing algaegrowth

• reduces the stirring effect of wind on the effluentsurface. This may otherwise stir up settled solidsand associated nutrients.

Plant selection and establishment is important.However, vegetation alone cannot ensure the successof a wetland system without the right quality of in-flowing effluent, system size and configuration.

Plant establishment problems can multiply if:

• planting occurs too late in the season

• there is insufficient or excessive water

• inappropriate soils or gravels are used

• plants are damaged by livestock or pukeko

• there is weed invasion and suppression of desirableplants.

Plants in a constructed wetland system may be floating, submerged or emergent. Systems using emergent plantsare recommended in New Zealand.

Emergent plants are rooted in the bottom soil or gravel and have their stems and leaves extending up, through theflow, to above the effluent surface (refer to Figure 3.10-9). The stems and roots filter out suspended solids and providesites for bacterial breakdown of the effluent.

Floating plants can be used in surface-flow wetlands. They float on the effluent surface with their roots extendingdown into the flow. The use of some floating plants in wetlands may be helpful.

Submerged vegetation is not widely used in constructed wetlands because it is shaded out by algae and isgenerally sensitive to anaerobic conditions.

Wherever possible, use native species and locally sourced plants. These are likely to do better in the local climate.Choose a selection of plants that are compatible (i.e. no single species is likely to out-compete and dominate.)

Do not grow trees on the wetland embankments as the roots will provide seepage lanes, weakening theembankment wall. However, plantings of flax and native toetoe on the embankments will provide habitatenhancement, bank stabilisation and weed exclusion.

Some guidelines for sourcing and planting wetland vegetation include:

• for best establishment, plant vegetation from spring through to early summer (i.e. between October andChristmas), provided appropriate water conditions can be maintained. Early spring planting will allowgood growth and coverage before the following winter. It is essential that water (fresh water or effluent fromthe aerobic pond) is available during plant establishment. Some plants will die back during winter or in a briefperiod of drought but will then re-grow from underground rhizomes. Additional attention to weed control isrequired in this case

• it is important to check plant availability well before the planting contract is let. Some plants may be outof stock due to their popularity during the spring season

• wetland species can be obtained as either small plants established from seed (e.g. 1-yr-old root trainer grade) oras bare-root rhizome cuttings with shoots trimmed to 200-300 mm. Both will establish well in the rightconditions. Do not bring plants in too far in advance. They must be maintained up until the time of planting incool, semi-shaded conditions and kept watered

• emergent wetland vegetation should initially be planted at a density of 4 plants per m2

• plants should be planted at 40-60 mm depth in the growth medium and be well firmed so that they are lessprone to uprooting and do not float out when water levels are raised

• immediately after planting, water levels in surface-flow wetlands should be raised to 50-100 mm above the soilsurface to optimise conditions for the wetland plants and suppress weed growth. Avoid flooding above theheight of the plant shoots. In subsurface wetlands, water levels should be maintained within 20 mm of thegravel surface during plant establishment. Use the pond water if there is a reliable flow at this time, orsupplement from another water supply

• when plants are well established, water levels can be raised in stages e.g. to 200-250 mm after one season’sgrowth and to a final depth of 300-400 mm after the second growing season. After this time, surface-flowwetland plants can be subjected to short periods of drought, but subsurface-flow plants are more dependenton water levels being maintained due to the poor water storage capacity of gravel.

Plant establishment may be a problem where birds, small animals and stock have access to the constructedwetland. Pukeko are a particular problem as they will uproot young plants and graze the growing tips.

To exclude pukeko, install a fence with 3 horizontal trip wires approximately 100 mm above the ground and 200mm apart. Gas bangers are sometimes effectively used to deter pukeko.

Stock need to be prevented from entering the wetland area. For these reasons it is recommended that theconstructed wetland be permanently fenced off.

Table 3.10-3 gives recommended wetland plants commonly used in New Zealand.

Table 3.10-4 gives plant species that should be avoided and eradicated if identified. They are either unsuitable aswetland species or classed as noxious weeds.

3.10.8 Constructed wetland maintenance

A major goal in constructed wetland design is to minimise and simplify the maintenance needed. Once plants havebeen established, wetlands require minimal ongoing maintenance. With wetlands, early identification of problemsis the key. Maintenance relates to:

• care of the pre-treatment system to ensure the pond system is discharging high quality effluent (refer to 3.8Pond system maintenance). Maintaining high quality effluent into the wetland will greatly prolong its life asexcessive solids will clog the wetland and raise the bed level, requiring overhaul and desludging

• inlet and outlet structures. Check that flows are evenly distributed across the wetland, adjusting T-pipes upand down until the flow is visually uniform (refer to Figure 3.10-8). Pipes should be inspected for blockages ordamage. End caps of both inlet and outlet pipes should be removed annually to flush the pipes. Outlet pipesshould be adjusted up or down to keep water levels at 300 mm for surface wetlands and just below the gravelsurface for subsurface wetlands. Water level adjustments may be required as sludge gradually accumulates

• embankments and fencing. Check for weed spread from the embankments and signs of erosion or damage.Graze outer embankments very lightly (with sheep if possible) but otherwise maintain stock exclusion

• ensure the wetland plants are growing well over the majority of the bed and check they are healthy.Plants will die off naturally in winter but stress at other times needs to be rectified. The most likely causeis drying out at times when cows are not milking or when ponds have been drained for desludging. Replantbare areas with new plants or those carefully thinned from dense areas to fill gaps

• ensure nuisance plants do not invade and dominate the wetland system. Such plants will need to beremoved either by hand weeding or for more extensive invasions by draining cells and physically removing theplants or applying a suitable selective herbicide. It may otherwise be possible to adjust effluent flow to drownout unwanted plants.

The following is a schedule of periodic maintenance tasks that need to be planned for.

Regularly

• Visually inspect vegetation for any signs of stress or invasion of unwanted species.

• Monitor the wetland area for pests. Pukeko may invade the wetland system, uprooting and eating wetlandplants. Areas can be fenced to keep out larger wildlife, but it is more effective to ensure sufficient plant speciesunattractive to foraging animals are established in the wetland. Control of animal pests such as possums, ratsand weasels, stoats or ferrets will also enhance bird life.

• Monitor the wetland area for odour problems. Standing, swampy water will cause odour problems. Byensuring good circulation and flow of effluent, dead zones will be reduced and odours should be minimised.

• Check the effect of the discharge on the receiving waterway.

• Check that the pipes running in and out of the ponds are not blocked and that in-flow is uniform acrossthe wetland.

• Adjust effluent levels to maintain them at the correct depth.

• Check that the fencing remains stock proof.

Six monthly to annually

• Repair and maintain inlet and outlet structures. Clean and remove plants around outlet pipe to provideaccess and guard against blockages.

• The wetland area may require desludging after ten years or move to remove accumulated sediments andassociated nutrients. This should be carried out in stages, not removing all the vegetation at a single time.Replanting should follow desludging.

3.10.9 Constructed wetlands – top tips to avoid trouble

• Does the existing initial treatment system operate correctly? If not, the constructed wetland cannot beexpected to work to its potential.

• Before installation, determine the likely expansions to herd and property size or intensification over thenext 10 years. Ensure sufficient wetland area is part of the design.

• Design the wetland to be of appropriate shape and depth to minimise short-circuiting and maximiseretention time.

• Ensure the wetland is adequately sealed to prevent water loss or groundwater intrusion.

• Assess the pollution risks associated with the failure of the wetland, should it be flushed during stormsor not operate to expectations.

• Install a drainage channel around the wetland to prevent water runoff from the land entering thesystem. Stormwater may carry pesticides, herbicides and create excessive loading.

• Do not let prohibited chemical materials enter the wetland. Many chemicals can affect the breakdown ofeffluent and the growth of wetland plants.

• Check availability of planting material early on in the planning process.

• Establish wetland plants in shallow water before gradually introducing higher effluent volume. Maintainadequate effluent flow to ensure year-round survival of wetland plants.

• Waterfowl can be deterred from destroying young plants by using temporary electric fencing, trip wiresand gas bangers. Fence to exclude stock.

• Regularly check the wetland plants and carry out maintenance on in-flow and out-flow structures.

3.10.10 Constructed wetland regulations

Food Safety and Dairy Industry RequirementsThe survival of various disease-causing micro-organisms during effluent storage and treatment is summarised in2.13.1 Food safety and dairy industry requirements. This section also outlines their requirements surroundingeffluent treatment and storage facilities. A constructed wetland system can be defined as such a facility and mustnot be sited within 45 m of the farm dairy facility.

Regional Council requirementsRegional Council concern is focused on the quality of effluent discharged into waterways following wetlandtreatment. Poorly treated and discharged effluent can cause elevated nutrient levels, suffocation and poisoning ofaquatic life, increased water turbidity and increased pathogenic micro-organism levels.

While some Regional Councils do not recommend this method, others view constructed wetlands as a usefuladdition to the traditional pond system, especially when required dilution in the receiving water is not available.

It should be noted that although constructed wetlands offer a possible additional treatment alternative, where theydischarge to a waterway a Resource Consent will still be required.

For region-specific regulations regarding the discharge of effluent to waterways check with your Regional Council.